- [Michelle] It's hot today and it feels even hotter because I'm in a city.

But scientists are experimenting with a common sunscreen ingredient to see if they can help with that problem.

Titanium dioxide is found in a lot of sunscreens and makeup and is also a new ingredient in this road rejuvenation treatment, which is used to extend the life of an asphalt road.

Asphalt is bound together by a substance called asphalt cement.

- Asphalt cement can be broken down into two major components.

Those are asphalt tenes.

Those are the dark, hard bodying parts.

And then you have the maltenes.

Those are the light oils and resins.

It's the light oils and resins that keep asphalt flexible and adhesive and slow down the aging inside the pavement.

- [Michelle] Asphalt rejuvenators put those maltenes back into the road, and the city of Raleigh, North Carolina, has been incorporating them into their road maintenance plan since 2004.

- Adding a pavement rejuvenator will hopefully get us five to seven years of expanded life throughout the pavement.

- [Michelle] Now they're adding a new ingredient to these treatments, titanium dioxide, to help mitigate what's known as the urban heat island effect.

- So the urban heat island is just an environmental phenomenon where urban areas tend to be hotter than the surrounding rural and suburban areas.

And basically, there are a few factors that cause this.

One of the main ones is that man-made materials tend to absorb heat and release it later on.

Roads are typically made out of asphalt, which is one of the materials, the kind of man-made materials that has the greatest capacity to absorb and re-radiate heat.

And because roads have such a large surface area in urban areas, if we can address the impact from roads, then we can have a really big impact on hopefully reducing the urban heat island effect.

- [Michelle] So how does the treatment work?

Applying it doesn't take long.

- So the process is going to take us anywhere from 30 minutes to an hour on any one road.

- When we first put down the material on the street with the distributor truck, it does have an indicator dye in it to where we know we're getting proper coverage on the road.

- Now, this is not a coating that's going to lay on top of the road.

It's a penetrant.

So that yellow color is only there during the application phase.

- Once the material is laid down, it starts to absorb into the roadway.

And with good wind, ambient temperature and sunlight, actually, it'll penetrate pretty quickly.

And once that happens, we're able to put slag material and vehicles are able to drive on it.

- [Michelle] Globally, cities are exploring using cool pavement technologies like this one to reduce urban heat.

- The goal of cool pavement is to try to do what we can to lower the impact of the heat island effect that's created by a hard surface.

We're putting a penetrant into the pavement that will lower that heat island effect by influencing the solar reflective index.

The solar reflective index is basically taking not just the solar reflectivity or reflectance that a material would make, but it's taking into other factors of how much energy is actually stored and held into that that surface.

So we're not changing the thermal capacity of the roadway.

What we're changing is the amount of heat energy that soaks into the pavement that can be radiated back out as the surrounding area begins to cool.

- [Michelle] As the sun shines on the treated roadway, titanium dioxide gets to work, first by physically refracting sunlight.

- So if you think of titanium dioxide as having a bunch of facets, like a bunch of little diamonds, we're putting that into the pavement.

And so as heat energy comes into the pavement, it's redirected in many directions, kind of like the sparkle of a diamond.

- [Michelle] There is also a chemical reaction occurring.

- So the titanium dioxide being a photocatalyst, UV light or the heat energy that comes into the road, it's going to excite the titanium dioxide.

When I say excite, basically it makes the electrons inside of that titanium dioxide move faster.

And then eventually one is going to jump out of the containment of the titanium dioxide.

And as it escapes, it's going to release energy.

Energy is not created or destroyed, right?

It's just changed from one form to another.

So the sunlight or heat energy is coming in, it's going to expend some energy, freeing electrons.

And there's a lot of titanium dioxide that we're putting into the roadway.

So there are a lot of electrons being expended constantly.

- [Michelle] Besides dispersing heat from the sun, pavement technology is excited about additional potential benefits having titanium dioxide in roadway treatments could provide, specifically related to pollution.

- As we drive on a roadway, our tires wear.

That's a microplastic.

And so the photocatalytic process has been shown to break down these plastics.

It also breaks down tailpipe emissions.

So nitrous oxide, sulfur oxides, volatile organics, they're coming out of the tailpipe.

Effectively, what we're doing is creating an oxidative or rusting.

So you think about oxidation on metal.

It's a rust.

So we're beginning to break down that pollution to give Mother Nature a little helping hand to start breaking down what's coming out of the tailpipe and what other pollution may be around the pavement.

- [Michelle] And rejuvenating a road is more affordable than resurfacing one.

- Cost to resurface, fully resurface a road could be upwards of $40 a square yard as to add this treatment to try to get five to seven years could be up to $2.50 per square yard.

- [Michelle] This is still new technology.

They've seen it work well in the lab and now need to better understand what happens when it's actually applied to roadways.

Part of that is measuring the impact treating a road with titanium dioxide has on the urban heat island effect in the surrounding area.

The city of Raleigh has been strategically choosing which streets to prioritize applying this treatment to based on measured heat data.

And scientists like Katherine are working to measure the treatment's effects.

- We have a pretty good sense that the coatings that they do, these cool pavement treatments will lower surface temperatures and we're interested in seeing if that is also contributing to air temperatures.

- Okay, so this is one of your sensor sites.

- Yeah, so this is one of 60 sensors that I have out in Raleigh and they look like this.

So this like white honeycomb is a radiation shield.

It protects it from weather and it protects the sensor from getting heated up from the sun rather than from the air temperature around it.

So inside is the sensor.

So there are these like little orange guys.

- [Michelle] Looks kind of like a Tamagotchi.

- Yes.

And it's got a tiny little white temperature probe inside.

And these are all mounted on utility poles and light poles all around the city alongside the streets.

- [Michelle] Sensors are mounted at around two meters height to measure what someone's experience would be when they're walking alongside the road.

What is the sensor actually measuring?

- So the sensor is capturing air temperature and humidity and then also calculating heat index, which is a better thermal comfort measure of how you would feel standing by the street.

I'm collecting 20 minute interval data.

So we're really getting a good picture of like the whole day, what it looks like in different weather conditions over a long period of time at all of these different locations.

- [Michelle] Katherine has been collecting data at these sites all summer and chose the locations based on streets that Raleigh planned to treat with titanium dioxide.

- So one of the things that Raleigh does is they apply this treatment to streets that were repaved three years ago or six years ago.

So we narrow down all of the other streets in Raleigh based on when they were repaved and then match the treatment streets to the potential comparison streets based on different street characteristics.

- [Michelle] Looking at things like street size, amount of shade and satellite data of land surface temperatures for the streets being treated, Katherine found similar streets that weren't being treated to use as a control.

- We have 30 sensors on treatment streets and 30 sensors on comparison streets that hopefully we can use to get a sense of what the impacts are.

- [Michelle] The results of Katherine's study and others like it will help show the impact that cool pavement technology can have when applied in an urban setting.

And although cool pavement may not solve urban heat issues on its own, it's part of the toolbox cities can use to reduce heat impacts to urban populations.

- So it can be like a really nice complement to something like tree planting.

You plant your trees and you expect that impact to grow.

You can do the cool pavements and you expect that impact to kind of happen right away, be a little bit more immediate, address any of the kind of heat concerns that are most pressing in the city and use other techniques to address kind of longer term heat.

- If you enjoyed that story, there's more where that came from.

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